The present invention relates to a cathode ray tube, and particularly, but not exclusively, to a display tube having a channel plate electron multiplier and electrostatic beam scanning at the input side of the electron multiplier.
British patent specification No. 2101396A discloses such a display tube. Display tubes having channel plate electron multipliers are particularly susceptible to contrast degradation due to electrons being scattered from the input surface of the electron multiplier and entering channels at a point distant from their point of origin. In the case of electrostatically scanned display tubes, particularly flat display tubes, it is not possible to produce a positively biased field at the input side of the electron multiplier to draw-off back-scattered electrons because this would conflict with the field conditions necessary to achieve proper scanning of the incident electron beam, these field conditions being created by deflection electrodes held at the same potential or a more negative potential than the multiplier input.
It is an object of the present invention to reduce the contrast degradation due to back-scattered electrons in cathode ray tubes having a channel plate electron multiplier and especially those having electrostatic beam scanning.
According to the present invention there is provided a cathode ray tube comprising an envelope having an optically transparent faceplate, and within the envelope, means for producing an electron beam, a channel plate electron multiplier mounted adjacent to, but spaced from, the faceplate, scanning means for scanning the electron beam across an input side of the electron multiplier so that the electron beam approaches the input side along a path which is inclined thereto, and means at said input side for limiting the acceptance angle of the electron multiplier.
The present invention is based on the recognition of the fact that when an addressing electron beam is deflected in the manner described then its angle of approach to the input of the electron multiplier falls within a narrow range. In contrast back-scattered electrons will approach the input dynode at any angle and the effect of limiting the acceptance angle of the electron multiplier will be to exclude a large number of the back-scattered electrons from entering the channels of the electron multiplier.
The scanning means may comprise a carrier member spaced from and arranged substantially parallel to the input side of the electron multiplier, the carrier member having thereon a plurality of adjacent, substantially parallel electrodes which in response to voltages applied thereto deflect the electron beam from a path between the carrier member and the input side of the electron multiplier, towards said input side.
The acceptance angle may be limited in a number of ways depending on the form of the electron multiplier. If the electron multiplier comprises a laminated stack of discrete dynodes and it is desired to physically restrict the acceptance angle then this can be done by mounting inclined vanes on the input dynode or mounting one or more apertured electrodes on the input dynode, the or each electrode being offset relative to the input dynode and/or each other so that the apertures in the electrode(s) form correspondingly inclined passages to their associated channels in the electron multiplier. The apertures in the or each electrode may be slanted.
Another way of limiting the acceptance angle is to reduce the number of secondary emitting electrons produced by back-scattered electrons by applying secondary materials to corresponding restricted portions of the peripheries of the convergent apertures in the input dynode. In this way, the addressing electron beam strikes the secondary emitting material and produces many secondary electrons whereas back-scattered electrons which will approach the input dynode at other angles will strike the untreated areas of the hole peripheries and will produce significantly fewer secondary electrons.
Back-scatter from the input of the electron multiplier can be reduced further by masking the area between the apertures of the input of the electron multiplier with a layer of a material having a low back-scatter coefficient which material preferably has a low coefficient of secondary emission. In the present specification by a low back-scatter coefficient is meant less than that of a smooth carbon layer and by a low secondary emission coefficient is meant a value less than 2.0 for electrons in the energy range 300 to 500 eV.
It has been found desirable that either the surface onto which the layer is applied or the layer itself is microscopically rough. This reduces significantly the number of back-scattered electrons produced.
The layer of low back-scatter material may be applied to the input (or first) dynode of the electron multiplier or alternatively to the vanes or the apertured electrodes which are mounted on the input dynode to restrict the acceptance angle.
The low back-scatter material may comprise black chromium, black nickel, black copper, optionally coated with a conductive layer, such as carbon, which has a low secondary emission and/or low back-scatter coefficient, or anodised aluminum onto which an electrically conductive coating is applied.
In the case of the electron multiplier being a glass matrix electron multiplier having continuous channels and input and output electrodes applied to the input and output surfaces thereof, the input electrode is arranged to extend into the channels such that the portion in each channel has an inclined end, the direction of inclination being substantially the same for all channels. Such an electron multiplier is mounted so that the electron beam proper strikes the glass wall of each channel causing a relatively large number of secondary electrons to be produced whereas back-scattered electrons entering channels from different directions strike the extended portions of the input electrode and relatively few secondary electrons are produced in consequence.